This invention relates to micro-CHP gas fired boilers. In this specification, a micro-CHP gas fired boiler means a gas fired boiler that delivers combined heat and power (CHP), i.e. both heat and electrical power, on a micro scale, i.e. with a total electrical power output not exceeding the total electrical power requirements of a domestic or small commercial building in which it may be installed.
A micro-CHP boiler is typically arranged to supply the heating requirements of a domestic or small commercial building and in addition to supply electrical energy which may supply the electrical demands of the boiler and heating system, and/or may supply a proportion of the electrical demand of the building, and/or may be connected via a meter to the electrical grid so as to reduce the total financial cost of energy consumed in the building by qualifying the boiler user for financial compensation for the small amount of power generated. By way of example, a micro-CHP boiler will typically produce a total electrical output of not more than about 10 kW, typically as little as 2 kW or so, although the principle can be applied on a somewhat larger scale.
A gas turbine assembly typically comprises a rotor including a turbine rotor, an air compressor rotor and a generator rotor all mounted to rotate together as one unit on a common rotor shaft. The rotor rotates at high speed and so high speed bearings such as hydrodynamic oil bearings or foil air bearings may be used to react axial forces applied to the shaft by the turbine and the compressor. For example, U.S. Pat. No. 5,827,040 discloses a gas turbine with a thrust runner mounted on the rotor shaft between the compressor and the turbine and having foil air bearings supplied with air bled from the compressor.
A particular constraint in micro-CHP gas fired boilers is the overall size of the boiler, which in a particularly popular configuration may be wall hung, for example, in a kitchen or utility space of a domestic building. It is known to incorporate a gas turbine into such a boiler to produce the electrical output, and to provide ducting to carry the exhaust gases from the turbine into the heat exchanger which supplies the space heating and/or hot water demand of the building. However, it is difficult to avoid increasing the size of the boiler casing to accommodate the gas turbine and ducting along with its ancillary components. In wall hung boilers for domestic installation it is particularly desirable to minimise the horizontal dimension of the boiler which projects from the wall so that the boiler is compatible with the dimensional constraints of a fitted kitchen.
Since the combustion chamber of a gas turbine operates at an elevated pressure, a fuel gas compressor is required to pressurise the fuel gas supply. It is known, particularly in industrial scale gas turbines, to mount a fuel gas compressor on the turbine rotor shaft. In most micro-CHP gas fired boilers however there is insufficient space for an extended rotor shaft, and so the fuel gas compressor is powered by an electric motor and mounted inside the outer casing of the boiler as a separate unit from the gas turbine assembly. This unit is typically large and expensive.
It is important for a micro-CHP gas fired boiler to be able to compete in the marketplace with gas-fired boilers which produce only heat and not electric power, both in terms of practical installation requirements, particularly the space required for the boiler, and in terms of the additional cost of integrating the gas turbine and ancillary components with the remaining components of an otherwise conventional gas fired boiler, when compared with the financial payback from the additional electrical generating capacity over its lifetime.
It is a general object of the present invention to more satisfactorily integrate a gas turbine assembly into a gas fired boiler to provide a micro-CHP gas fired boiler, in particular by ameliorating the impact on cost, overall dimensions, and ease of manufacture.